Could the "impassable" overlay then be used to expand a map progressively?
By creating a big map with most of it covered by "impassable" and shroud=yes (but revealed on the "passable" part of the map) and then removing the "impassable" overlay when we want to expand the map?
Since if I understand correctly the shroud is cleared by the line of sight which itself is actually the possible movement of the unit, it shouldn't show impassable terrain?

On another topic I liked your Dead Water campaing very much (even though I'm stuck on the second scenario, playing at Hard level) and especially the Merman Younglings (and the Brawler, for his Stun ability which is very interesting tactically). I'm working on a revision of HttT (already added a scenario and working on others), with a second scenario involving Mermen just after Bay of Pearls: would you consider a good idea to put Younglings (and letting them be promoted to Brawlers) there or is it better to let players discover them later in Dead Water?

Could the "impassable" overlay then be used to expand a map progressively?

I guess it could. An impassable overlay would keep the units from revealing what was on the other side of it, but when it was removed, the units would still only reveal the part of the map that they could reach in one turn. If I understand your idea correctly, the [remove_shroud] tag is a better way to do it. That's pretty much what it's made for.

Consider a map divided in half by an impassable river. The initial goal is to liberate and hold a village by the river for several turns while a bridge is built. You have a special unit (Engineers) which must occupy the village for a morning-to-evening cycle. If they do so, modify the map to place a bridge over the river. Add Shroud and Fog if you want. If the opponent can see the bridge going up, they probably would have some units positioned to hold their side and maybe even storm across to take out your Engineers.

Alternatives might be a detachment of Dwarves building a tunnel from a village through a mountain, or cutting a road through a narrow pass.

My point being: with a little thought, you can often design the map and the scenario so that it makes sense you can't get there from here, until something happens, in a way which does not involve simply denying movement.

But, as beetlenaut said, your goal is simply to avoid exposing details until there is a unit nearby, Fog and Shroud are there exactly for that purpose.

The fact that unexpected tags are ignored without a warning is often annoying in this way. I wonder if there couldn't be a --wml-debug mode that prints log messages about every tag that was parsed but not used.

In 1.15.0 this will be --validate-addon <your-addon-folder-name>. (Okay, so that's slightly different from what you describe, but for this purpose it would have about the same effect.)

Of course the remove shroud tag will be used when the impassable overlay will be removed, but adding the overlay means that bringing units near it will not reveal anything, so effectively one begins with a small map and this map is expanded (for all sides) through events that remove both the overlay and the shroud.
I get from your answers that it should work? I'll have to test it then.

BTW Celtic Minstrel, is there a release date for 1.15 (in stable version I mean)?

No, 1.16 will not be out in about a month. 1.15.0 might be, but it also might not be. There are a lot of new WML features and a few new core units, but I'm unaware of any major new features and in particular unaware of anything new that's directly relevant to players, so it's definitely not enough to make a 1.16.

would be great to add some more basic units to wesnoth.. like catapult, ballista, ram, elephant.. musketeers, couple more ships, steamship maybe... i mean just a tiny library of the very basic units that any medieval game should have.. selectively taken from add-ons.. wouldnt take much space, but would be very useful.. for no need to download add-on games.